在多列上计算 DISTINCT [英] Counting DISTINCT over multiple columns

问题描述

有没有更好的方法来做这样的查询:

Is there a better way of doing a query like this:

SELECT COUNT(*) 
FROM (SELECT DISTINCT DocumentId, DocumentSessionId
      FROM DocumentOutputItems) AS internalQuery

我需要计算此表中不同项目的数量，但不同项目超过两列.

I need to count the number of distinct items from this table but the distinct is over two columns.

我的查询工作正常，但我想知道是否可以仅使用一个查询(不使用子查询)获得最终结果

My query works fine but I was wondering if I can get the final result using just one query (without using a sub-query)

推荐答案

如果您想提高性能，可以尝试在两列的散列值或串联值上创建持久计算列.

If you are trying to improve performance, you could try creating a persisted computed column on either a hash or concatenated value of the two columns.

一旦它被持久化，如果该列是确定性的并且您使用的是健全的"数据库设置，则可以对其进行索引和/或可以在其上创建统计信息.

Once it is persisted, provided the column is deterministic and you are using "sane" database settings, it can be indexed and / or statistics can be created on it.

我相信计算列的不同计数将等同于您的查询.

I believe a distinct count of the computed column would be equivalent to your query.

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